Well this is one of the secret and hack of airplanes. You can open the aircraft’s locked lavatory from outside. The lock is being designed so that incase a kid gets locked inside or someone tries to get himself locked purposely for various reasons than a flight attendant can open the door from outside. Here is how it can be done :
1.On the lavatory door you will find a “Lavatory” sign.
2. Lift the “Lavatory” sign.
3. Slide the knob on to Unlock position.
There you Go !!